The present invention relates to a method and apparatus for treating a nonwoven web of thermoplastic fibers.
As used herein, the term xe2x80x9ctreatmentxe2x80x9d includes everything that can be achieved with local plastification and/or melting of a nonwoven textile of thermoplastic fibers that accompanies the effect of hot raised embossing. In other words, treatment refers in particular to thermal bonding of the nonwoven due to local adhesion of the fibers (as discussed in German Patent OS 1 808 286 and U.S. Pat. No. 3,478,141). Treatment also refers to formation of perforations, with melting of the fiber zone located below a raised embossing area, and displacement of the melted mass. A suitable calender for performing this operation is disclosed in German Patent 34 16 004 A1.
In that patent, an embossing roller made of steel and a smooth counter-roller made of steel rotate at the same circumference velocity and form a roller pair. A nonwoven textile made of thermoplastic fibers is guided through the roller pair. The raised parts of the embossing roller rest against the counter-roller with their faces. Because they are heated, the raised areas partially or completely melt the parts of the nonwoven textile that are held between the rollers, and bond the nonwoven by bonding the fibers, or displace the melted mass to the edge of the face of the embossing surface. At the edge of the face, the melted thermoplastic material solidifies and forms bonding zones that edge the holes formed by the displacement, and, at the same time, stabilize the nonwoven.
After leaving the roller nip, the web of the nonwoven textile is rather stiff and board-like, because of the many regions of compact material that have melted together.
WO 98/07907 discloses mechanically softening a non-woven structure produced by xe2x80x9cflash spinning.xe2x80x9d However, no information, in detail, is provided as to how to do this.
The present invention is based on the object of developing the described process so that a nonwoven textile made of thermoplastic fibers and xe2x80x9ctreatedxe2x80x9d in the manner described has a softer feel. A softer feel is desirable in many situations.
In accordance with that object, a web is passed through a roller nip between an embossing roller and a counter-roller. This causes local plastificationxe2x80x94i.e. meltingxe2x80x94in the nonwoven at the locations of raised embossing areas on the embossing roller. The areas of plastification in the nonwoven that have solidified after leaving the roller nip are broken.
By breaking them, the rigid regions that are formed by local plastification of the thermoplastic material are broken down. This significantly reduces the hard feel of the nonwoven textile.
In accordance with another aspect of the present invention, the nonwoven web can be positively cooled after leaving the roller nip and before being broken. This makes the thermoplastic material of the nonwoven textile more brittle and the breaking effect more accessible.
As used herein, xe2x80x9cpositivexe2x80x9d cooling is cooling brought about by a cooling device in contrast to cooling that results from merely giving off heat into the surroundings.
A calender for use in the present invention has a rotating, heated embossing roller made of steel. A counter-roller acts together with the embossing roller. Means for passing the web through a roller nip formed between the embossing roller and the counter-roller is provided.
Also provided are means for pressing the embossing roller and the counter-roller together, in such a way that the embossing in the web produces local plastification in the nonwoven at the locations of raised embossing areas as the nonwoven passes through the roller nip.
Following the roller nip is a device for breaking the areas of plastification that have solidified after leaving the roller nip.
In first embodiment of a breaking device, a third roller is provided. The third roller forms a three roller calendar, and is unheated or positively cooled.
The advantage of the third toller is that the two rollers of the calender and the breaking unit are integrated into a single assembly.
A particularly advantageous embodiment has three rollers aligned in one plane. The outer two rollers are deflection-controlled rollers and hold the middle roller between them. This arrangement allows uniformity of treatment of the nonwoven textile in the roller nip over the width of the web. In this embodiment, the web is cooled on the looping path of the non-heated or actually cooled counter-roller.
In a further development of the device, a device for positive cooling of the treated nonwoven web before breaking is provided.
It has proven to be practical if the web is thermally bonded before passing through the calender so that the fibers of the nonwoven are no longer loose, but rather grouped in a geometrically defined structure before they run into the roller nip. This prevents the undesirable adhesion of individual fibers to the hot embossing roller and an accumulation of thermoplastic material on the roller.